In many production process systems, a fluid having suspended particles is used, either as a raw material, an intermediate product or a final product. Examples may be found in widely differing areas, such as pulp or paper industries, pharmaceutical industries, food processing, building material fabrication, etc. Common for many of the processes is that the inherent properties, size or concentration of the suspended particles are of crucial importance for the final product. Therefore, there is a general desire to find methods for analyzing the properties of the particles in a fast, accurate, safe, cheap and easy manner in order to predict the final product quality and to be able to control the processing steps accordingly.
Some basic types of measurement philosophies exist for process fluids; “off-line”, “on-line”, “at-line” and “in-line”.
The classical off-line procedure is to extract samples of the process fluid for analysis in a laboratory. However, in this way only a part of the process fluid is analyzed, and the possible feedback of such an analysis is generally slow.
An analysis method suitable for providing data for control purposes has to be performed in direct contact with the actual process fluid flow.
To speed up the off-line procedure (up to 5-10 times) an on-line procedure with automatic sampling systems have been developed in which measurements based on, e.g., optical measurement techniques are used. Typically such systems operate by diverting a small portion of the process fluid into a special pipe or volume. One example being the PQM system (Pulp Quality Monitor) from Sunds Defibrator, which measures freeness, fiber length and shive content in a pulp suspension. A common problem with all off-line and some on-line and at-line methods is that only a part of the flow is measured. The properties in such a diversion flow may differ from the main flow. TCA (Thermomechanical pulp Consistency Analyser) from ABB AB measures the consistency of the pulp. The system is using fiber optic techniques. Other similar systems are the Smart Pulp Platform (SPP™) available from ABB, and “Fiber Master” developed by the Swedish pulp and paper research institute (STFI).
In-line methods, which operates directly on the entire process fluid without extracting fluid into a special test space, are generally faster than off-line methods and can reduce some of the problems listed for these methods. However, mechanical devices have to be inserted in the process line in order to extract the flow sample, which may disturb the main flow and which makes maintenance or replacement work difficult. Furthermore sensors may be contaminated, or the flow may be contaminated by the sensors.
An alternative to use optical or electromagnetic waves is to use mechanical (acoustical) waves. This has several advantages. Acoustic waves are environmentally friendly and also unlike electromagnetic waves they can propagate in all types of fluids.
In the article “Ultrasonic propagation in paper fiber suspensions” by D. J. Adams, 3rd International IFAC Conference on Instrumentation and Automation in the Paper, Rubber and Plastics Industries, p. 187-194, Noordnederlands Boekbedrijf, Antwerp, Belgium, it is disclosed to send ultrasonic beams of frequencies between 0.6 MHz and 15 MHz through a suspension of fibers and the attenuation as well as the phase velocity can be measured as a function of frequency, It is by this possible to obtain information about fiber concentration, size and to some extent the fiber state. However, an elaborate calibration procedure is necessary in order to make the method operable.
In “Pulp suspension flow measurement using ultrasonics and correlation” by M. Karras, E. Harkonen, J. Tornberg and O. Hirsimaki, 1982 Ultrasonics Symposium Proceedings, p. 915-918, vol. 2, Ed: B. R. McAvoy, IEEE, New York, N.Y., USA, a transit time measurement system is disclosed. The system measures primarily the mean flow velocity and tests from various pulp suspensions are described. Doppler shift measurements are used to determine velocity profiles. A frequency of 2.5 MHz was used.
In U.S. Pat. No. 3,710,615, a device and method for measuring of particle concentrations in fluids is disclosed. An acoustic wave of one wavelength is emitted into a fluid containing particles. The amplitude of the acoustic signal is registered and the attenuation of the acoustic signal is deduced. Based on this attenuation, a particle concentration is determined. One embodiment where two frequencies are used is also described. Frequencies of 1 MHz and 200 kHz are mentioned.
In U.S. Pat. No. 5,714,691, a method and system for analyzing a two phase flow is presented. An ultrasonic signal is introduced in a two phase flow and the echo signals are registered by a set of sensors. The flow rate and flow quality is determined based on these measurements. Furthermore, the results are used for regulate the flow. Excluding flow characteristics are discussed.
In the French patent publication FR 2 772 476 a method and a device for monitoring phase transitions are described. The method uses measurements of wave propagation velocities to estimate viscoelastic properties of e.g. milk products, which are subjects to phase transitions. Preferred frequencies are above 10 kHz.
In the international patent application WO 99/15890 a method and a device for process monitoring using acoustic measurements were disclosed. Inherent acoustical fields in the system (up to 100 kHz) are recorded indirectly via wall vibration measurements on a conveyor line, through which a fiber suspension flows. The recordings are graded by a data manipulation program according to predetermined characteristics and a vibration characteristics is generated. Stored vibration characteristics related to earlier recordings are compared at each recording for correlation to the properties of the suspension. The recorded vibrations can be used for controlling the process in a suitable way, for raising alarms at fault situations or for showing changed tendencies.
In the international patent application WO 00/00793, measurements of fluid parameters in pipes are presented. A speed of sound is determined by measuring acoustic pressure signals at a number of locations along the pipe. From the speed of sound, other parameters, such as fluid fraction, salinity etc. can be deduced. Frequencies below 20 kHz are used. Preferably, the method operates only on noise created within the system itself. However, an explicit acoustic noise source may be used.
Since the method used in the above patents is based on a method which makes use of inherently appearing vibrations, or other noise signals, a number of problems result. One being that not only will sound generated in the fluid be picked up but also vibrations from mechanical sources, e.g., pumps, connected to the fluid. This leads to large amounts of disturbances, which increases the amount of averaging or over-determination. Furthermore, since there are no control of the source process methods for suppressing disturbances are difficult to apply. In addition the suggested method must be calibrated for each individual site, since the inherent vibrations are site dependent. This last aspect is a considerable practical limitation since it will cause very large losses in production upon installation.